[improve docs]: Timestamp pallet (#1435)

This PR improves the docs for the Timestamp pallet by following our
[Documentation
Guidelines](https://github.com/paritytech/polkadot-sdk/blob/master/docs/DOCUMENTATION_GUIDELINE.md

)
more closely.

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Co-authored-by: Juan <[email protected]>
Co-authored-by: Francisco Aguirre <[email protected]>